Transmit filter

Abstract

A transmit filter ( 100 ) receives a stream of data symbols (DT_TX) at a baseband symbol clock rate. An available clock (FREF) is used to generate sample points for producing a generating an oversampled signal. The available clock is independent from the baseband symbol clock, and does not need to be an integer multiple of the clock. Upon identifying a start sequence in the data stream, a phase tracking circuit ( 106 ) is used to determine a current position relative to the baseband symbol clock. A state circuit ( 104 ) stores the last three, or more, data symbols. Based on the last three data symbols (which determines the shape of the curve for the current data symbol) and the current position (which determines the current position on the curve), a filter circuit ( 108 ) generates a sample point.

1. A transmit filter for generating an oversampled signal from a stream of data symbols generated responsive to a symbol clock, comprising:
 circuitry for receiving the data symbol stream; 
 phase tracking circuitry, responsive to a reference clock generated independently from the symbol clock, for maintaining phase information relative to the symbol clock; and 
 sample generating circuitry for generating samples responsive to said phase information. 
 
   
   
     2. The transmit filter of  claim 1  wherein said sample generating circuitry generates samples at an active edge of said reference clock. 
   
   
     3. The transmit filter of  claim 2  wherein said sample generating circuitry generates samples on each clock cycle of said reference clock. 
   
   
     4. The transmit filter of  claim 2  wherein said sample generating circuitry generates samples on selected clock cycles of said reference clock. 
   
   
     5. The transmit filter of  claim 1  wherein said reference clock comprises the output of a frequency divider. 
   
   
     6. The transmit filter of  claim 1  wherein said reference clock is selectable from two or more clock signals. 
   
   
     7. The transmit filter of  claim 1  wherein said phase tracking circuitry comprises circuitry for adding a predetermined value to a stored value on each clock cycle of said reference clock. 
   
   
     8. The transmit filter of  claim 7  wherein said predetermined value is a ratio between a frequency associated with said symbol clock and a frequency associated with said reference clock. 
   
   
     9. The transmit filter of  claim 1  and further comprising circuitry for storing a current data symbol and a predetermined number of preceding data symbols. 
   
   
     10. The transmit filter of  claim 9  wherein said sample generating circuitry comprises circuitry for generating a sample point responsive to said phase information, said current data symbol and one or more of said preceding data symbols. 
   
   
     11. The transmit filter of  claim 10  wherein symbol data for generating a sample point is defined by a plurality of transfer function curves. 
   
   
     12. The transmit filter of  claim 11  wherein symbol data for one of said curves is stored in a memory and symbol data for other of said curves is derived from said symbol data for said one curve. 
   
   
     13. The transmit filter of  claim 11  wherein the symbol data for said one curve comprises a power of two number of data points. 
   
   
     14. The transmit filter of  claim 11  wherein said memory stores symbol data for multiple sets of transfer curves. 
   
   
     15. The transmit filter of  claim 11  wherein symbol data for multiple sets of transfer curves are stored in respective memories. 
   
   
     16. The transmit filter of  claim 1  and further comprising circuitry for identifying an approximate center of a data symbol. 
   
   
     17. The transmit filter of  claim 16  and further comprising circuitry for tracking an approximate center for each data symbol in said stream independent of the symbol clock. 
   
   
     18. A method of generating an oversampled signal from a stream of data symbols generated responsive to a symbol clock, comprising the steps of:
 receiving the data symbol stream; 
 maintaining phase information relative to the symbo in clock in response to a reference clock generated independently from the symbol clock; and 
 generating samples responsive to said phase information and said reference clock. 
 
   
   
     19. The method of  claim 18  wherein said sample generating step comprises the step of generating samples at an active edge of said reference clock. 
   
   
     20. The method of  claim 19  wherein said sample generating step comprises the step of generating samples on each clock cycle of said reference clock. 
   
   
     21. The method of  claim 19  wherein said sample generating step comprises the step of generating samples on selected clock cycles of said reference clock. 
   
   
     22. The method of  claim 18  and further comprising the step of generating the reference clock through a frequency divider. 
   
   
     23. The method of  claim 18  and further comprising the step of selecting the reference clock from two or more clock signals. 
   
   
     24. The method of  claim 18  wherein said step of maintaining phase information comprises the step of adding a predetermined value to a stored value on each clock cycle of said reference clock. 
   
   
     25. The method of  claim 24  wherein said predetermined value is a ratio between a frequency associated with said symbol clock and a frequency associated with said reference clock. 
   
   
     26. The method of  claim 18  and further comprising the step of storing a current data symbol and a predetermined number of preceding data symbols. 
   
   
     27. The method of  claim 26  wherein said sample generating step comprises the step of generating a sample point responsive to said phase information, said current data symbol and one or more of said preceding data symbols. 
   
   
     28. The method of  claim 27  wherein symbol data for generating a sample point is defined by a plurality of transfer function curves. 
   
   
     29. The method of  claim 28  and further comprising the steps of storing symbol data for one of said curves is stored in a memory and deriving symbol data for other of said curves from said symbol data for said one curve. 
   
   
     30. The method of  claim 28  wherein the symbol data for said one curve comprises a power of two number of data points. 
   
   
     31. The method of  claim 28  wherein said storing step comprises the step of storing symbol data for multiple sets of transfer curves in one or more memories. 
   
   
     32. The method of  claim 18  and further comprising the step of identifying an approximate center of a data symbol. 
   
   
     33. The method of  claim 32  and further comprising the step of tracking an approximate center for each data symbol in said stream independent of the symbol clock. 
   
   
     34. A transmit filter for generating an oversampled signal from a stream of data symbols generated responsive to a symbol clock, comprising:
 circuitry for receiving the data symbol stream; 
 phase tracking circuitry, responsive to a reference clock, for maintaining phase information relative to the symbol clock; and 
 sample generating circuitry for selectively generating samples responsive to said phase information and said symbol clock. 
 
   
   
     35. The transmit filter of  claim 34  wherein said sample generating circuitry generates samples on randomly selected cycles of said reference clock. 
   
   
     36. The transmit filter of  claim 34  wherein said sample generating circuitry generates samples on deterministically selected cycles of said reference clock.